Moving target indication



eC 9, 1952 D. E. sUNsTr-:IN 65,243

MOVING TARGET INDICATION Filed Dec. 5, 194e 2 SHEETS- SHEET 1 AGE/V75 r Dec 9, 1952 D. E. sUNsTElN 2,521,243

MOVING TARGET INDICATION Filed Dec. 5, 194e 2 SHEETS- SHEET 2 Patented Dec. 9, 1952 ZZLZiB MOVING TARGET INDCATION David E. Sunstein, Cynwyd, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporationof `llennsylvania Application December 5, 1946, Serial No. 714,289

4 Claims. l

This invention relates to object motion indicating systems. More specifically it relates to such systems of the reectionr type in Ywhich there may be relative motion between an energy transmitter (which forms an essential part of the system) and the medium in which the energy is propagated to cause it to tranverse the space between the transmitter and target objects from which it is reected. The invention is applicable to an vobject motion indicating system of the socalled storage type in which reflections from target objects are stored and subsequently compared with reiiections later received from the same objects, whereby motion of certain'of the objects in the interim is detectable. It also is applicable to systems of the non-storage -type such, for example, as the one described in my eopending application, Serial No. 694,297, filed August 3l, 1946, in which motion of target objects-is detected by noting alterations in the characteristics (i. e. frequency components) of pulses of oscillatory energy upon reiiection from the objects. The intention is particularly applicable in such systems when the velocity of the transmitter, relative to the medium through which the transmitted energy is propagated, is of a magnitude such as to produce an appreciable difference in the frequencies of oscillations produced in the medium from those of the transmitter signals used to induce such oscillations. This may, for example, be the case in a system of the sort customarily used in the location of underwater objects. Such systems usually employ oscillations in the so-called supersonic range (i. e. 'from 10 to 100 kilocycles) to propagate, in water, waves whose velocity of propagation may be of the order of the velocity of sound in water. It is not unusual for the velocity of a vessel carrying transmitting equipment to be of the order of a percent or more of this velocity of propagation. Changes vin the velocity of the transmitter with reference to the water will therefore p-roduce substantial differences inthe frequencies of the disturbances created in the water by the transmitter. These differences, in turn, will produce like diiferences in the characteristics of reflections of the propagated Waves from objects in the water. maintaining substantially constant the frequencies of the Waves propagated lin the water, these differences in the characteristics of reflections from objects vcan be minimized or substantially eliminated.

Under what arenow regarded as normal conditions, this problem is notso `serious in the case Iof radar and similar systems in which the velocity of propagation (i. e. `of radio frequency kwaves in air) approaches 'the velocity of licht. At vthe present time velocities of even the fastest vaircraft Will amount to but a very small fraction of a percent of the 'velocity of light. Hence the variation in the frequency of radio Waves, owing to changes in velocity of an aircraft in which thetransmitting equipment is situated, will, as a rule, be insignicant so far as their tendency to produce variations in the characteristics of reiiections from moving objects is concerned. It is to be understood, however, that although the linvention may not be needed under present conditions for radar purposes, it is equally applicable in principle to such systems, and might, for eX- ample, prove useful in aircraft or missiles whose velocities are exceedingly high.

Before proceeding with a detailed exposition of the invention it will be helpful to describe in outline certainvtypical systems of the sort to which it is applicable. There will first be considered an object motion indicator system of the socalled storage type above referred to. Such a system, Whether for supersonic or radio frequency application, may comprise a transmitter of pulsed oscillatory energy adapted to effect the propagation .in a medium, Asuch as Water or air, of waves whose frequencies will correspond in general to the frequencies of oscillations generated in the transmitter. Reflections of such waves from target objects are intercepted by a suitable receiver and stored until the receipt of subsequent reflections from -the lsanle objects produced in response vto a subsequently 'transmitted energy pulse. If one of the objects is in motion relative to its surroundings, the frequency of energy reiiected from it will differ from the transmitted frequency and this, in turn, will give rise to a difference in phase of successive reflections from the same object. These reflections beat either with the return from fix-ed obi-ectsor with the output of a localcoherent oscillator 4in the receiver to produce pulses of differing amplitude in resp-onse to successive reflections from the moving object. The subsequently received reflections are compared in amplitude with the stored reiiections'from the same targets, and any difference noted will beindicative ofthe shift in frequency of the reflections produced vby target motion. In order that successive refie'c'tions from a xed target may not differ as to their characteristics and prevent distinguishing between-moving and non n1oving objects, some systems of .this sortemploymeans Afor relating .the phases of successive transmitted pulses. For this purpose there may be employed a so-called coherent oscillator, which oscillates continuously and which is used to determine the phase of each successive pulse of oscillatory energy generated by the transmitter.

The system just outlined constitutes an object motion indicator of the more conventional type. More recently it has been possible, as set forth in my aforementioned copending application, to provide a system by means of which motion of a target object can be detected from the characteristics of the reflection from the object of a single pulse of transmitted energy. Such a system possesses the advantage that it eliminates the need for a coherent oscillator and, as well, the need for means for storing a given reflection until a subsequent reflection from the same object is received for comparison with it. As set forth in my copending application above identined, there are transmitted pulses of energy comprising a relatively narrow band of frequency components. Reflections of such pulses from moving objects are found to contain frequency components outside the transmitted band, which components are produced either by Doppler or other effects. In the receiver the components thus resulting from reflection are separated from the transmitted components, for example, by means of a band-rejection lter having a rejection band coinciding with the band of transmitted frequencies. Thus the presence, in a received reflected pulse, of frequency components which are outside this band constitutes an indication that the target object, from which the reflection is received, is in motion.

Because of the advantages, under certain circumstances, of a system of the sort just referred to over systems of the storage type, I have chosen to describe the present invention as embodied in a system of the non-storage type.

A primary object of the invention is to provide an object motion indicator of the reflection type, in which the characteristics of reflections from objects are substantially independent of the velocity of a transmitter with reference to a medium in which waves are thereby propagated to produce said reflections.

Another object is to provide, in an object motion indicator of the reflection type, means for propagating waves in a medium by means of a transmitter located in a vehicle movable with reference to the medium and for maintaining certain characteristics of said waves substantially invariant despite changes in the velocity of the vehicle with reference to the medium.

Another object is to provide, in an object motion indicator of the reflection type, means, including a receiver located in a vehicle movable with reference to a medium, for transducing waves propagated in the medium so that at least certain of the characteristics of the waves thus transduced are maintained substantially invariant despite changes in the velocity of the vehicle with reference to the medium.

Still another object of the invention is to provide an object motion indicator system, comprising a transmitter and a receiver located in a vehicle movable with reference to a medium, for propagating waves in said medium and for receiving reflections of said waves from target objects in such a manner that at least certain characteristics of said received reflected waves are substantially independent of variations in the velocity of said vehicle with reference to said medium but vary in response to the motion f target objects producing them, whereby they are indicative of motion of said objects.

Other objects and advantages of the invention will become apparent from a consideration of the following description together with the accompanying drawings in which:

Figure 1 is a generalized diagram of a nonstorage type target motion indicator according to the invention, and

Figure 2 is an explanatory diagram.

Referring now to Figure l, master oscillator and keyer I may be a source of time-spaced, pulse-modulated electrical oscillations at a carrier frequency fo in the supersonic range of from 10 to 100 kilocycles. As set forth in my aforementioned copending application, each pulse may comprise sideband frequency components which are of substantial magnitude only within a relatively narrow range (e. g. cycles at a carrier frequency of 20 kilocycles). As hereinbefore mentioned, and as more fully set forth in my copending application, the restriction of the transmitted band in this Vmanner makes readily possible the detection of components outside the band, which are produced by translation of the original components upon reflection from moving objects. The recurrence frequency of the pulses, as will be apparent from the subsequent description, determines the maximum range of object against which the system is operable. Thus, for example, if the system is to be used in seawater against targets at a maximum range of 2,000 yards, the pulse repetition period may be 2.5 seconds.

The pulsed oscillations from master oscillator and keyer I are supplied through connection 2 to a mixer 3. Also there is supplied to mixer 3, through connection 4 from a variable frequency oscillator 5, a signal whose frequency is controlled as will hereinafter be set forth and has a value f1 at the time of occurrence of a pulse of energy from oscillator I. heterodyned, and from the resultant there is derived, by means of a filter which may form a part of the mixer, the difference frequency fo-fi which is pulse modulated at the same rate as the signal fo from master osillator and keyer I. These pulses are supplied through connection 6 to a power amplifier 1 and thence through connection `8 and T-R (transmit-receive) switch 9 to transducer II. T-R switch 9 may comprise any suitable means, such as a commutator, Which is controlled so as to connect transducer I I to the output of power amplier 1 during the occurrence of pulses therein which are to be transmitted, and at other times to connect transducer I I to mixer I6 to permit the transfer of received pulses therebetween. Transducer I I, for the frequencies here exemplified, is preferably of the magnetostriction type. Transducers of this sort are well known in the art and need not be described 1n detail. Transducer Il, in response to the electrical pulses thus supplied to it, is adapted to propagate compression waves in the water which separates the transducer from various target objects, such as the one represented at I2. Depending upon the velocity of the ship in which the apparatus of Figure 1 is installed, the frequencies of the waves propagated in the waterV will differ from those of the oscillations supplied to transducer II. For example, for Va certain velocity of the ship with reference to the water, oscillations at the carrier frequency fo-fi may result in the propagation of compression waves in the water at a carrier frequency fo. These are transmited through the water along broken The two signals are linef-I3 to the-targety I2. Ifthe` target isinmotion, there-will'; be produced, inthe-` wavesy reflected therefrom,` component Waves Whosecarrier. frequency. differs from thatofthe transmitted waves; Wavefwhich. returns along broken line It vfrom target object I2 to transducer `Ii may havel ak carrier'frequency of fa-I-Af, Where the magnitude andsign of Af depends on the'speed `and direction of-travelof the target. Transducer. II `is also adapted to interceptY these Areflected .Waves and to convertr them into electrical signals.

These signals, if there is relative motion between:

the Water-and theA vessel in which the equipment is situated;l will. comprise frequency components attributable-to such -motion` in addition to those attributable-to the componentsgcomprising the Waves originally propagated. The carrier fre-V quency of the vreceived signal may, for example, berepresented by the vexpression fe-l-fz-I-Af- This expression takes into Vaccount the-.fact .that the velocity relativeto the-water at the time of arrival of'reflections at transducer l l differ from the-relative velocity atthe time of transmission. Such. variaticnrmay, for example, be caused by av change in `the forward velocity of the ship orA At all times ex by pitching or rolling thereof. cept during .I the transmission of pulses from power'ampliier. Ito transducer E! through-connections S and '-I, TRswitch s may be adapted tosupplyreceived reflected signals .from transducer II through connections Iii and l5 to mixer I5. rIhe latter isalso supplied, through connection I'I from `another mixer I3, with signals Whose frequency is equal to fa-z. These signals, in turn; are fobtained by mixing,V in mixer I3, the signal from variable frequencyoscillator 5, supplied tothe mixer through connection Ie, With a signal of fixed frequency f3, supplied through connection 2i! from local oscillator.y El. reasons which Will later be pointed out, the frequency of the signal from oscillator- 5 is fz at the time of arrival of a reflection fromy a targetA object. The mixermay include filter means -for selecting thedifference frequency ,f3-f2. This signal is supplied through connection i? to mixer I6 Where it is'heterodyned with-the signal supplied through connection I5, whose carrier frequency is fn-I-f2-i-Af. The resultant difference signal has a carrier frequency of foa-,fa-I-Af. The effect thus produced is the same as if the incoming signal had first been shifted in frequency by an amount f2 equal to the shift produced in receptionby motion of the transducer relative to they water, andthe resultant signal had then been beatvvvith the local oscillator signal f3 to yield aV signal at a suitable intermediate fre-a quencyjo-I-fg-l-nf. This intermediate frequency signal is supplied to anl intermediate frequency amplier 22, which may also be adapted to funce tion as -a band-rejection filter.

present in the-pulse-of energy originally transmitted, and Vwhich have not been shifted by reflection from a moving target.

nal which have been shifted owing to target motion.. These components, as has already been noted, .are indicative of :target motion may be supplied through connection E3 to any suitable l and conventional form of indicatcria, `to indie cate the presence, inthe water surrounding the vessel in :which the equipment is located; offob.- Y

jects in .motion relative .-thereto..

Thus; for example, the reflected AForA It should be ca pablo -of rejecting the carrier frequency f0+3- and sidebands thereof corresponding'to thoselt should, hovv-A evenpass thecomponents of the transmitted sig- Transducerfl I Ais mounted for orientation about a -pvot 25;' and its..orientation mayg'be systematically; controlled by; means :of aY motor 2 6 1 through; a. mechanical .linkage comprising Acrank :2 -`I .cone necting"rod"28"and A.lever arm `il rse as Y to .causery it successively to propagatewaves in different directions.y To thisend; theradiation charactereisticl offtransducer II is preferably` directional.

There is -also provided a water speed indicator` 30 .which is-ada-pted to indicateythe-instantaneousv velocity'of th'evesself relative to the water-vin; thedirection in which .thetransducer I I is orient ed.' To this yend-theorientation of indicatonty may becontrolled in synchronisnrwith the orien-. tation -.of transducer I I, ,likewise by; means; of motor 26; through the linkagegcomprising :crank 2l; connecting rod:28'. and leverrarm 3|. The water Aspeed indicator-3G- maybe relatively `simple inform, comprising,l as shown., a propeller 32- and .an .electrical generator 33 mechanically coupledy tothe propeller by means of .pulleys-34 fand 35 and belt36..

As the speed of rotation of propeller 32 varies' in response tov variations'in the speed'v ofzwater' past it, the-.output'lof generator 33 will-vary. This outputis-.suppliedthrough connectionBI tot variable frequency-oscillator E to control its fre-'- quency. This .controlling action maybef madesuoh that upon theoccurrence of each'lpiulseof oscillations at theoarrier `frequency fn, the output of the variable frequency oscillator 5 will be of the required frequency lf1 .to beat IWiththe carrier frequency fu toyield pulsedv oscillations -atthe frequencyl foefirsuchas vto. maintain substan` tially constant the frequency fo of waves .-propa.- gated. in.the.-Water-. Likewise, at the. timeof. arrival of a reflection. from av target object, .the output of variable frequency oscillator 5 will be of the frequency f2 necessary to eliminate, from the received signals, thefrequency shift vproduced bymotion of'transducer, II relative tothevvater.4 Thus, regardless .ofy variations in the Velocityof such motion, the .resultant 4interniediate frequency signal fo-I-fs-I-Af will remain constant, and

, the frequency componentstherein correspondingA to those in the .transmittedsi-gnal canbe substantially eliminated .by means. of .a .fixedband-frejection filter, which isihere included in ythezintermediate frequency amplirler22.1

As `already' mentioned,r it is. essential for tar.- getlrnotion detection purposes to .maintainsubstantially constant the frequencies of Wavesprollagatedina. medium p-riorto their reflection fromv target objects. It..will .beviapparent that. the .system of Figure l inaccordance with. the invention vis adapted to achieve thisresult.

The .principlesof the 4.invention and the mode-- of S. operation of the embodiment illustrated in Figure 1 will bgmore .clearly understood by ref: erenceetotheexplanatory frequency diagram of.' Figure2. Herethefinitewidths of the frequency` bandsare indicated,.though notto scale, but .no effort is madeto indicate the relative magnitudesy ofthe several frequency components Within each.

b`and At 'SIlisrepresentedthe band of frequencies generated by masteroscillato'r and keyer. I y of .Figure 1A and..having. .a.carrier frequency fu.

Were-.these signalsto .be ,supplied .directly to .the transd-ucerl I `.Without modication by the action of fmixer, the carrier frequency of the signalsv propagated in-.the-,Waterwould be fo-I-fi .as-repref sented-v at .5 I 1 Uppn..reflection ,from-.axed.tar-. get `;oloject,.no ...change vLint .these signals wouldbef produced.. and ...theiband of .reiiected components..7 would be as. represented 'iat ...52.y However, if `.the.

target is moving toward the transmitter at a predetermined velocity, these components will be shifted in frequency by an amount Af, as represented at 53. Upon arrival at the transducer, and depending upon the relative velocity between the transducer and the water at that time, there would occur a further shift in frequencies of reiections from both fixed and moving targets, which, for example, might be equal to f2. The received components from a fixed target would then appear as represented at 54, while those for a moving target would appear as represented at 55. Either of these signals might be shifted downward in frequency by an amount f1-1-f2, in which event the resultant components for the fixed target would appear at 56 While those for a moving target would be displaced therefrom by an amount Af, as represented at 57. The signals from moving targets could then be separated from those of xed targets by means of a bandrejection filter centered at frequency fo. Alternatively, of course, the receiver filter could be centered at the frequency Jo-l-fi-l-fz, in which event it would be unnecessary to effect a downward shift in frequency of the received signals, It will be noted however, in the mode of operation just described, that the band of frequencies propagated in the water would vary up and down in frequency from the position shown at 5| depending upon the relative velocity between the transducer and the water. This would result in undesired variations in the characteristics of reflections, which, as already pointed out, would render it diiiicult, if not impossible, to distinguish between fixed and moving targets. Thus, this mode of operation would be unsatisfactory except where the relative velocity between the transducer and the water remained substantially constant.

According to another mode of operation, the transmitter carrier, prior to transmission, might be shifted downward in frequency by an amount fi-l-fz to yield a signal having a carrier located at fo as represented at 58 in Figure 2. This presupposes that the velocity of the transducer relative to the Water at the time of arrival of a reflection from a predetermined object could be predicted in advance in order to permit the determination of the magnitude of the frequency shift f2. The carrier frequency of the signal propagated in the water would then be shifted upward by an amount f1 as represented at 59. The reflections respectively from fixed and moving targets would occupy the bands 6E) and 6I. The received signals, for fixed and moving targets respectively, would be further shifted in frequency by an amount f2 and would occupy bands 52 and 63. These signals could be supplied directly, and without need for further shifting, to the lter having a rejection band centered at frequency fo. Again this mode of operation would be subject to the objection that the band of frequencies 59 occupied by the signals propagated in the water would move up and down depending upon the relative velocity between the transducer and the water.

According to the present invention, however, the signals at the transmitter are first shifted downward prior to transmission by an amount f1, so as t0 cause them to occupy a band centered at frequency fo as represented at '64. Frequency ,f1 varies according to the relative velocity of the transducer with respect to the water, so that the band of frequencies occupied by the signals propagated in the water remains substantially xed at the frequency fo, as represented at 65. Upon reflection from a fixed target object, the band of refiected components will be unaltered in frequency as represented at 66. However, for moving targets the band will be shifted as represented at B1. In reception a further shift will occur for reflections from both fixed and moving targets Whose respective bands will appear as shown at 68 and 69. Then, in accordance with the invention, the received signals are shifted by an amount f2 which varies in accordance with the velocities of the transducer relative to the water, so that the resultant signals for a fixed target occupy the band 56 and may be eliminated by means of the band-rejection filter, while those for a moving target occupy the band 51 and are substantially unaffected by the band-Y rejection filter.

Thus it will be seen that, by shifting the frequency of the signal supplied to the transducer for transmission by an amount proportional to the relative velocity between the transmitter and the medium, the frequency of waves propagated in the medium can be maintained essentially constant, so as to avoid variations in the components produced in object-reflected waves. Furthermore, by shifting the received signals by an amount proportional to the relative velocity at the time of reception, the resultant signal may be maintained centered on the rejection band of a fixed filter adapted to eliminate the components corresponding to those originally transmitted and to select only those components which are indicative of target motion.

It may be reiterated, as will be apparent to those skilled in the art, that although the invention has been described With particuluar reference to a supersonic object detection system, in which it appears to have its maximum usefulness at the present time, it is equally applicable, though perhaps not as much needed, in systems operating at much higher frequenciesi. e. radar and the like. It will also be appreciated that the invention is susceptible of embodiment physically in forms other than the one here shown. Accordingly, the invention is to be regarded as subject only to the limitations 1mposed by the appended claims.

I claim:

l. An object motion indicating system for use in a vehicle movable with reference to a medium, said system comprising: a transducer responsive to signals applied thereto for propagating waves in said medium, means for supplying signals to said transducer, means for varying the frequency of signa-ls supplied to said transducer as a function of the velocity of said vehicle with reference to said medium, areceiver of reflections of said propagated Waves from target objects, means for controllably altering the frequencies of components of said received reflections as a function of the velocity 0f said vehicle with reference to said medium to maintain the frequencies of certain of said components within a predetermined frequency band, an indicator, and means for supplying to Asaid indicator said altered frequency components of said refiections which lie outside said predetermined band, said last-named means comprising fixed frequency band rejection filter means constructed and arranged to reject frequency components within said predetermined frequency band.

2. An object motion indicating system for use in a vehicle movable with reference to a medium,

said system comprising: means for propagating Y Waves in said medium, means responsive to the velocity of said vehicle with reference to said medium for controlling said first named means to maintain the frequency of waves propagated in said medium substantially constant, a receiver of reflections of said propagated waves from target objects, means responsive to the velocity of said vehicle with reference to said medium for modifying said received reflections to eliminate substantially therefrom alterations in the frequencies thereof from the frequencies of reflected propagated waves in said medium occurring in the reception of said waves owing to motion of said vehicle with reference to said medium, an indicator, and means for supplying certain components of said modified received reflections to said indicator, said last-named means including fixed frequency band rejection filter means constructed and arranged to reject certain other components of said modied received reflections lying Within a predetermined fixed frequency band.

3. In a Wave transducing system for use in a vehicle movable with reference to a medium in which are propagated Waves comprising frequency components lying both Within and outside a predetermined rst frequency band, means for transducing said propagated waves to produce other Waves, means responsive to the velocity of said vehicle with reference to said medium for controllably altering the frequency components of said other waves to maintain within a predetermined second frequency band those components of said other Waves which are produced in response to the components of said propagated waves lying within said first frequency band, a utilization device, and means for supplying to said utilization device said altered frequency components which lie outside said second frequency band, said last-named means comprising xed frequency band rejection filter means constructed and arranged to reject frequency components within said second-frequency band.

4. An object motion indicating system for use in a Vehicle movable with reference to a medium, said system comprising: means in said vehicle for propagating waves in said medium, means responsive to the velocity of said vehicle with reference to said medium for controlling the frequencies of said Waves to cause reflections thereof from moving target objects to comprise frequency componets which are distinguishable from the frequency components in reflections from target objects which are substantially xed, means responsive to the velocity of said vehicle with reference to said medium for translating said first-named frequency components to frequencies Within a predetermined frequency band and for translating said last-named frequency components to frequencies outside said band, an indicator, and means for supplying to said indicator said translated components which lie outside said predetermined frequency band, said last-named means comprising fixed frequency band rejection filter means constructed and arranged to reject frequency components Within said predetermined frequency band.

DAVID E. SUNSTEIN.

REFERENCES CTED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,204,826 Schiessler Nov. 14, 1916 1,528,210 Hammond Mar. 3, 1925 1,747,041 Alexanderson Feb. 11, 1930 1,801,466 Townsend Apr. 21, 1931 1,864,638 Chilowsky June 28, 1932 2,418,490 Turner Apr. 8, 1947 2,438,580 Schuck Mar. 30, 1948 

